Revving high losing power..etc..

Hi everyone
Its been a long time since ive been here..since all your help the last time now i have problems again..i cant pinpoint it... Here it goes..
I have a 96 jetta gls automatic..the thing at idle runs smooth I rev it in park and it revs smooth.
I drive the thing and it revs high then low then high then evens out and seems fine.
The car does not go above 50 mph and is getting terrible gas mileage..
I just replaced the O2 sensors last year..I thought at first that was the problem again because auto zones thing said so..both of them gone..
there was a problem with the exhaust pipes near the rear 02 sensor, I thought that was throwing it off...after it was fixed problem got worse.
Maybe the catalytic converter????? Hell if i know.. Thanks in advance for anyones help!!!!!! AJ
ps.. it doesnt do this as bad in 3rd gear...but still stays below 50 mph

If the engine isn't misfiring & there isn't a check engine light then I'd go with the catalytic also. Most exhaust shops can check them out fairly easily.

yes the check engine light is on..however..I redlined in park and it backfired a couple times and now its fine...so i think a tune up and a cat. check is in order...do you guys think the cat. is bad??????? someone mentioned about the transmission slipping but i dont know.... thanks Jettn